1. Filed of the Invention
The present invention generally relates to a driving force transmission apparatus used in an image forming apparatus such as a copying machine, a printer, a facsimile, or a composite type machine made of these appliances. More specifically, the present invention relates to an improvement in such a driving force transmission apparatus for transmitting driving force by wearing, or tensioning an endless flat belt on a plurality of tension members, and also relates to an improvement in an image forming apparatus with employment of this driving force transmission apparatus.
2. Description of the Related Art
Driving force transmission apparatus which are utilized in image forming apparatus such as a printer and a copying machine own such specific characteristics that structural defects of these driving force transmission apparatus may directly induce image defects. Under such a circumstance, various sorts and higher requirements such as high engaging rates, high transmission rate, and suppression of rotation fluctuations.
In order to realize the high engaging rates and the high transmission rates, or in order to achieve the better low rotation fluctuation performance as driving force transmission apparatus of image forming apparatus according to the related art, there has been proposed a technique transmitting driving force to an image carrier by using helical gears (see, for instance, Japanese Laid-open Patent Applications No. Hei-9-80840, and No. Hei-5-72862).
Also, another technique using belts equipped with teeth has been proposed as driving force transmission member since employment of the belts equipped with teeth can realize lower rotation fluctuation performance than the employment of the helical gear.
Furthermore, in order to realize a lower rotation fluctuation than that realized by employing the belt equipped with teeth, another technique using a helical gear belt has been proposed (see, for example, Japanese Laid-open Patent Applications No. Hei-9-160332 and No. Hei-10-26903).
In general, in the case that a helical gear is employed as a driving force transmission member, the engaging rate is made large easily, as compared with such a case that a spur gear is employed as this driving force transmission member, and also, a meshing between a drive gear and a driven gear is carried out slowly. As a result, it can be found that meshing vibrations which are transferred to this driven gear can be considerably reduced. However, in such a case that a gear is employed as such a driving force transmission member, such a technical problem cannot be avoided. Namely, rotation fluctuations are produced by a back lash.
In other words, when the drive gear is meshed, or engaged with the driven gear so as to rotary-drive this driven gear, the teeth of the drive gear is mutually made in contact with the teeth of the driven gear only for a predetermined time duration. However, after this time duration elapses, the teeth of the drive gear and the teeth of the driven gear are brought into non-contact states until the next teethes are meshed with each other, which implies a back lash (play). As a result, a back lash produces vibrations when the next teethes of these drive/driven gears are engaged with each other, and the driven gear constitutes a factor by which periodic rotation fluctuations occur (namely, engagement between teethes of gears are repeated).
This back lash cannot be in principle avoided in the case that gears are used. In such a case that gears are employed as a driving force transmission member of an image forming apparatus, a driven gear is vibrated by such a gear engagement to be easily moved only by a movement corresponding to a back lash under non-constraint state. As a result, even such a small vibration force caused by the engagement vibration causes the periodic concentration (density) fluctuations to be produced in an output image.
Also, even when a helical gear is employed, a total number of meshed teeth cannot be largely increased, as compared with such a case that a spur gear is used. As a consequence, when a deformation problem as to teeth is considered, the helical gear is required to be manufactured by using such a material having a certain high hardness especially in a meshing contact portion thereof.
However, in such a case that a drive gear made of a material having a high hardness is engaged with a driven gear made of a material having a high hardness, since a portion capable of absorbing vibrations produced by this gear engagement is not present in a driving force transmission path (namely, within transmission path in case that several rotating members are driven by gear train), meshing vibrations which are produced by meshing the drive gear with the driven gear are not attenuated, but are directly transferred to the driven gear. As a result, there is another technical problem that periodic concentration fluctuations are produced in an output image.
On the other hand, in a driving force transmission apparatus with employment of a belt equipped with teeth, since the belt equipped with the teeth which is meshed with a pulley is made of such a material as a rubber material having superior flexibility, it is so expected that vibrations produced by engaging the pulley with the belt equipped with the teeth are smaller than those produced by the gears. However, as a result of the actual measurement, there is substantially no difference between the vibrations produced by the belt equipped with the teeth and the gears, as explained as follows:
That is to say, FIG. 31 is a diagram for representing rotation fluctuations in case that both a spur gear belt and a spur gear are employed as a driving force transmission member.
As apparent from FIG. 31, even when the spur gear belt is employed, rotation fluctuation results thereof are obtained which are not different from those of the spur gear.
Apparently, a rotation fluctuation can be improved by narrowing a pitch of teeth. However, when this pitch of the teeth is excessively narrowed, a so-called xe2x80x9cteeth skipping phenomenonxe2x80x9d caused by an increased load occurs, so that gears cannot be actually driven. Therefore, a great improvement could not be expected. As a result, even when the spur gear belt is employed, it is impossible to avoid an occurrence of concentration fluctuations in an output image.
FIG. 32 represents a relationship between a concentration fluctuation allowable value and a rotation fluctuation of an image carrier drum in an image forming apparatus such as a printer.
In this drawing, such a rotation fluctuation level of the image carrier drum, at which a concentration fluctuation contained in an output image can be recognized, is equal to approximately 0.3% in a speed variation rate of xcex94V0xe2x88x92p (%) which constitutes an index of such a rotation fluctuation. When such a speed variation rate larger than, or equal to this fluctuation level occurs, there is a problem as to concentration fluctuations of the output image. As a consequence, the engaging vibration levels produced in the gears and the belt equipped with the teeth constitute a very serious problem in view of this concentration (density) fluctuation.
In other words, as to the rotation fluctuation requirement as the image forming apparatus, very high levels are required. Even when a helical gear belt is employed, a so-called xe2x80x9cteeth skipping phenomenonxe2x80x9d occurs in a similar manner to that of the spur gear belt. It is practically difficult to achieve such an improvement that the rotation fluctuation level of the spur gear belt shown in FIG. 31 is reduced lower than, or equal to the allowable value.
As a consequence, as the techniques according to the related arts capable of solving such a technical problem, for instance, the following driving force transmission apparatus has already been proposed (see, for instance, Japanese Laid-open Patent Application No. Hei-7-319254). That is, as a driving force transmission apparatus for moving outer peripheral planes of a plurality of image carrier drums by the same move amounts, respectively, an endless-shaped flat belt is worn between a drive pulley and a driven pulley in order to transfer the driving force.
In this type of driving force transmission apparatus, since the driving force is transferred between the flat belt and the pulley (drive pulley and driven pulley) by way of friction force, in principle, such meshing vibrations which are produced by meshing the gear with the belt equipped with the gear are not produced between the flat belt and the pulley. As a result, this technique according to the related art can effectively prevent the periodic concentration fluctuation from being produced in the output image, which occurs in such a case that the gear and the belt equipped with the gear are employed.
However, in this sort of driving force transmission apparatus according to the related art using the flat belt, since the driving force transmission between the flat belt and the pulley is realized by utilizing the friction transmission, another technical problem newly occurs, namely, a slip occurs between the flat belt and the pulley.
In this case, FIG. 33 is a graph indicating a relationship between an average rotation speed of a driven pulley and load torque in a driving force transmission apparatus with employment of a flat belt.
As indicated in FIG. 33, the average rotation speed of the driven pulley is rapidly lowered when the load torque exceeds a limit value. This reason is given as follows: That is, while the normal slip amount is similarly increased in connection with an increase in the load given to the driven pulley shaft (namely, driven shaft), when the load becomes larger than, or equal to a certain limit value, the slip between the flat belt and the drive pulley, or the slip between the flat belt and the driven pulley is rapidly increased, so that the average rotation speed of the driven pulley is largely lowered.
When the driving force is transferred in the vicinity of the load amount of the driven pulley shaft under such a condition, the speed of this driven pulley is brought into unstable condition while time has passed. As a result, color shifts (color deviation) and/or transfer fluctuations occur in an output image, so that the normal image forming operation cannot be carried out. In the worst case, the image forming apparatus is stopped or malfunctions.
Also, in order to improve the limit value of the load torque, it is advantageous to increase the belt initial tension. In other words, since the belt initial tension is increased, depression force used to depress the belt against the pulley is increased. As a result, since the friction driving force is increased, the limit value of this load torque is increased.
However, in such a case that a rubber belt and/or a resin belt is employed, rigidity of the belt itself is low, and thus, high tension cannot be applied thereto. As a result, the use of metal belts may be conceived in order to secure rigidity of a driving force transmission system and also to obtain stable driving force. However, since a friction coefficient between such a metal belt and a pulley is extremely smaller than a friction coefficient between either a rubber belt or a resin belt and a pulley, as represented in FIG. 33, a limit value of a load given to a driven pulley shaft cannot be largely improved. Even when doubled tension was applied to these rubber and resin belts, these belts could not be driven by achieving a target load amount of an image forming apparatus.
Also, in order that a limit value of load torque is improved by employing a metal belt, in such a case that very large belt initial tension is applied to this metal belt, shafts for supporting pulleys are flexed. As a result, the alignment of the respective pulley shafts is shifted, and the metal belt is largely meandered. Accordingly, since the flat belt was rubbed under large force with respect to the belt edge guides provided on the pulleys, distortions were produced in belt edge portions, so that the flat belt was driven under unstable condition.
As a consequence, under such a driven load condition predictable in an actual image forming apparatus, there is fatal defect, for instance, stable image forming operation cannot be realized. This metal belt employment could not also constitute the satisfactory solution.
Furthermore, a winding angle of a belt to a pulley is increased, and whereby belt depression force given to the pulley by tension can be increased. As a result, increasing of the belt winding angle to the pulley is effective with respect to a slip of a belt.
For instance, as indicated in FIG. 34, the following technique has already been proposed, i.e., while a driving pulley (not shown) is provided in a coaxial manner with respect to a plurality of photosensitive drums 500 (500Y, 500M, 500C, 500K) as a driving force transmission apparatus for the plural photosensitive drums, a flat belt 510 is wound on this driving pulley. In addition to both a drive tension pulley 501 and a driven tension pulley 502, several pieces of auxiliary tension pulleys 511 to 515 are provided, so that the winding angle of the flat belt 510 with respect to such a pulley for driving the photosensitive drums 500 can be secured to be large (for example, see Japanese Laid-open Patent Applications No. Hei-7-319254, No. Hei-10-111586, and No. Hei-10-161384). Incidentally, in FIG. 34, a reference numeral 505 denotes a belt unit used in an intermediate transfer operation, or used to transport paper.
However, in this type of driving force transmission apparatus, a large space is necessarily required so as to tension the belt (flat belt 510), and also, the supporting members for supporting the auxiliary tension pulleys 511 to 515 are necessarily provided. Such an arrangement is not preferably employed in view of compact/low cost aspects.
The present invention has been made to solve the above-explained technical problems, and therefore, has an object to provide a driving force transmission apparatus and an image forming apparatus with employment of this driving force transmission apparatus. That is to say, in such a driving force transmission apparatus in which an endless-shaped flat belt is worn over a plurality of tension members, while eliminating a transmission error of driving force which is caused by slips produced among the endless-type flat belt and the tension members such as pulleys, the driving force can be transferred under stable condition even under such a condition that a heavy load is given to a member to be driven.
According to an aspect of the invention, there is provided an image forming apparatus comprising: a drive source for producing driving force; an image carrier driven by said driving force; and a driving force transmission apparatus for transmitting the driving force produced by said drive source to said image carrier, wherein said driving force transmission apparatus comprises: an endless-shaped flat belt having a plurality of through holes along a travel direction thereof; and a rotation member having a plurality of projections to which said plural through holes of the flat belt are fitted.